Retinal transplantation represents a rational approach for the treatment of blinding diseases of the retina. However, there are significant obstacles that must first be overcome before one can contemplate such interventions in a clinical setting. Among the most daunting barriers to successful grafting of new cells to the retina are 1) the capacity of the immune system to recognize and destroy grafted tissue placed into the eye; and 2) the ability of grafted tissue to survive, differentiate, and form functional connections with the host neuroretina. We have recently made new observations regarding 1) the role of microglia in the immunogenicity of retinal transplants; and 2) the immunogenicity of RPE cells. There have also been significant advances in the field of neural stem cell biology. We have used these immature, multipotent cells to overcome one fundamental barrier to successful retinal transplantation, the achievement of morphological integration of grafted tissue in the mature, diseased mammalian retina. In this application, we propose a series of experiments that will address what we consider to be the two key issues in the field of retinal transplantation; 1) immunobiology of retinal grafts; and 2) integration of grafted tissue with the host retina. We will 1) analyze the role of microglia in the immunogenicity and vulnerability to rejection of neuronal retinal transplants placed orthotopically and heterotopically; 2) analyze the immunogenicity of RPE cells as grafts and their vulnerability to rejection when placed orthotopically and heterotopically; 3) analyze the immunogenicity and vulnerability to rejection of CNS derived stem cells grafted orthotopically and heterotopically; and 4) study the fate of retinal stem cells implanted intraocularly. The anticipated results of these experiments will allow us to develop strategies aimed at producing retinal transplants that survive and form functional connections in the eyes of rodents with retinal dystrophies. We believe the demonstration of functionally integrated grafts, which also survive for long periods of time in an allogeneic setting, are vital and necessary prerequisites to further studies in higher animals. While all the studies proposed in this application involve the use of rodents, the knowledge gained will have relevance to humans, and will offer insight that might someday allow us to approach retinal transplantation in patients with blinding diseases of the retina.